The present invention relates to impingement cooling of a gas turbine nozzle band side wall in a design where the weld joint between the nozzle segment cover and the nozzle side wall is remote from the nozzle wall exposed to the hot gas path and particularly relates to a method of controlling the side wall thickness of the nozzle band to facilitate cooling thereof.
In current gas turbine designs, nozzle segments are typically arranged in an annular array about the rotary axis of the turbine. The array of segments forms outer and inner annular bands and a plurality of vanes extend between the bands. The bands and vanes define in part the hot gas path through the gas turbine. Each nozzle segment comprises an outer band portion and an inner band portion and one or more nozzle vanes extending between the outer and inner band portions. In current gas turbine designs, a cooling medium, for example, steam, is supplied to each of the nozzle segments. To accommodate the steam cooling, each band portion includes a nozzle wall in part defining the hot gas path through the turbine, a cover radially spaced from the nozzle wall defining a chamber therewith and an impingement plate disposed in the chamber. The impingement plate defines with the cover a first cavity on one side thereof for receiving cooling steam from a cooling steam inlet. The impingement plate also defines, along an opposite side thereof and with the nozzle wall, a second cavity. The impingement plate has a plurality of apertures for flowing the cooling steam from the first cavity into the second cavity for impingement cooling the nozzle wall. The cooling steam then flows radially inwardly through cavities in the vane(s), certain of which include inserts with apertures for impingement cooling the side walls of the vane. The cooling steam then enters a chamber in the inner band portion and reverses its flow direction for flow radially outwardly through an impingement plate for impingement cooling the nozzle wall of the inner band. The spent cooling medium flows back through a cavity in the vane to an exhaust port of the nozzle segment.
The cover provided each of the outer and inner band portions is preferably welded to the corresponding nozzle side wall. In prior designs, the weld joint between the cover and the nozzle side wall was disposed at a radial location between the nozzle wall and the spline seal between side walls of adjacent nozzle segments. In that location, the weld was exposed to the high temperature gases in the hot gas flow path and was very difficult to cool. Thus, weld joint fatigue life was significantly reduced due to its proximity to the hot gas path. Moreover, the location of the weld was not optimum for manufacturing repeatability and was very sensitive to manufacturing tolerances. The weld joint was characterized by variable wall thicknesses which increased the stress at the joint, decreased low cycle fatigue and limited the life of the parts. The wall thickness at the weld after machining is also a variable which could not be tolerated in the manufacturing process.
In a current nozzle segment design, the weld joint between the cover and nozzle side wall is on the side of the spline seal remote from the nozzle wall exposed to the hot gas path. That is, the weld joint between the cover and the nozzle side wall of the outer band is located radially outwardly of the spline seal between adjacent outer bands while the weld joint between the cover and the nozzle side wall of the inner band is located radially inwardly of the spline seal between adjacent inner bands. This reduces the temperature of the weld joints during turbine operation, reduces the stresses across the joints, both thermal and mechanical, eliminates any requirement for machining after welding and results in joints of constant thickness and higher fatigue life. The location also leads to improved machinability and tolerance to weld defects.
To provide that weld location, undercut regions adjacent the side walls of the nozzle segment bands are formed. Particularly, each undercut region includes a side wall or edge of the nozzle segment and an inturned flange extending inwardly from and generally parallel to the nozzle wall and spaced from the nozzle wall. Cooling the nozzle band side wall or edge, however, is quite difficult in view of the undercut region which spaces the side wall or edge a substantial distance from the impingement plate which, in turn, reduces the effectiveness of impingement cooling the segment side wall.
In accordance with the present invention, improved side wall fabrication and cooling is provided. Particularly, with the weld joint between the cover and the nozzle side wall located remotely from the hot gas path through the turbine, side wall cooling is facilitated by controlling the thickness of the side wall to a very tight tolerance. As will be appreciated from the foregoing, the side walls of each nozzle segment are very difficult to cool due to the large impingement gap in the undercut region, i.e., the substantial distance between the apertures of the impingement plate nearest the side wall and the side wall per se. The side wall is also not robust to manufacturing processes. This design is very dependent upon the casting process for the nozzle segment and the weld or other distortions that may occur during processing of the segment. If the side wall thickness is too thick, this results in reduced low cycle fatigue due to increased thermal strains on the segment. Increased stresses would also be introduced in this area or other areas of the nozzle segment. Side wall thickness variability is also a problem because post-cast machining operations may leave the wall too thick or too thin or perhaps even remove a portion of the wall. If the wall becomes too thick, reworking the segment is typically not allowed and the segment would have a greatly reduced part life. Also, the wall thickness might be so thick that the part could not be used. A thin wall, similarly as a thick wall, would cause stresses to either increase at the side wall location or in other areas of the segment. As a consequence, it has been determined that side wall thickness must be maintained within very tight tolerances in order to maintain appropriate cooling within the design parameters.
In accordance with a preferred embodiment of the present invention, the side wall thickness of the nozzle segment is controlled by preferably premachining a step in each of the side walls of the nozzle along the outer surface thereof prior to welding the cover to the nozzle segment. The step in the side wall is based on the inside surface position of the side wall, i.e., the datum for the machining is the internal wall surface. A thermal barrier coating (TBC) is applied in a relatively thick layer in the step by masking past the premachined surfaces. Subsequent to welding the covers to the nozzle segments, the side wall is finally machined, i.e., the thermal barrier coating is machined and serves as a buffer for and an accommodation to manufacturing tolerances to afford a predetermined side wall thickness after final machining. Thus, if the cast side wall of the nozzle segment is out-of-tolerance and after machining the TBC is thick, the TBC affords added protection to the side wall as it reduces the thermal gradient through the metal. If the TBC after final machining is relatively very thin or non-existent along the premachined step, due to out-of-tolerance formation of the side wall, this too is acceptable due to the purge of the cavity, i.e., the impingement cooling, and the fact that the wall is of known thickness. Thus, the manufacturing process provides the thermal barrier coating as a means to absorb or accommodate manufacturing tolerances to provide a very controlled wall thickness which will greatly enhance the low cycle fatigue of the segment and reduce any need to scrap the segment by reason of the part being out of tolerances.
In a preferred embodiment according to the present invention, there is provided a method of manufacturing nozzle segments having nozzle bands and vanes extending therebetween, the nozzle bands having side walls adjacent one another when the nozzle segments are assembled for use in a gas turbine, comprising the steps of applying a thermal barrier coating to at least one side wall of each nozzle band and machining one side wall with the thermal barrier coating serving as a buffer to accommodate manufacturing tolerances in the segment thereby controlling the side wall thickness of the nozzle segment.